Post-fire Survival Research Articles

A burning question: what are the risks and benefits of mammalian torpor during and after fires?

Although wildfires are increasing globally, available information on how mammals respond behaviourally and physiologically to fires is scant. Despite a large number of ecological studies, often examining animal diversity and abundance before and after fires, the reasons as to why some species perform better than others remain obscure. We examine how especially small mammals, which generally have high rates of energy expenditure and food requirements, deal with fires and post-fire conditions. We evaluate whether mammalian torpor, characterised by substantial reductions in body temperature, metabolic rate and water loss, plays a functional role in survival of mammals impacted by fires. Importantly, torpor permits small mammals to reduce their activity and foraging, and to survive on limited food. Torpid small mammals (marsupials and bats) can respond to smoke and arouse from torpor, which provides them with the possibility to evade direct exposure to fire, although their response is often slowed when ambient temperature is low. Post-fire conditions increase expression of torpor with a concomitant decrease in activity for free-ranging echidnas and small forest-dwelling marsupials, in response to reduced cover and reduced availability of terrestrial insects. Presence of charcoal and ash increases torpor use by captive small marsupials beyond food restriction alone, likely in anticipation of detrimental post-fire conditions. Interestingly, although volant bats use torpor on every day after fires, they respond by decreasing torpor duration, and increasing activity, perhaps because of the decrease in clutter and increase in foraging opportunities due to an increase in aerial insects. Our summary shows that torpor is an important tool for post-fire survival and, although the physiological and behavioural responses of small mammals to fire are complex, they seem to reflect energetic requirements and mode of foraging. We make recommendations on the conditions during management burns that are least likely to impact heterothermic mammals.

Physiological and behavioral responses of an arboreal mammal to smoke and charcoal-ash substrate

The recent observation that torpor plays a key role in post-fire survival has been mainly attributed to the reduced food resources after fires. However, some of these adjustments can be facilitated or amplified by environmental changes associated with fires, such as the presence of a charcoal-ash substrate. In a previous experiment on a small terrestrial mammal the presence of charcoal and ash linked to food restriction intensified torpor use. However, whether fire cues also act as a trigger of torpor use when food is available and whether they affect other species including arboreal mammals remains elusive. To evaluate whether smoke, charcoal and ash can act as proximate triggers for an impending period of food shortage requiring torpor for mammals, we conducted an experiment on captive sugar gliders (Petaurus breviceps), a small, arboreal marsupial, housed in outside aviaries under different food regimes and natural ambient conditions. When food was available, fire simulation via exposure to smoke and charcoal-ash substrate caused a significant earlier start of activity and a significant decrease in resting body temperature. In contrast, only when food was withheld, did smoke and charcoal-ash exposure significantly enhance torpor depth and duration. Thus, our study not only provides evidence that fire simulation does affect arboreal and terrestrial species similarly, but also suggests that smoke and ash were presumably selected as cues for torpor induction because they indicate an impending lack of food.

Damage and mortality patterns in young mixed conifer plantations following prescribed fires in the Sierra Nevada, California

High-severity wildfires increasingly influence forests in the western United States. Extensive research has identified preventative practices including mechanical and prescribed fire treatments to reduce wildfire severity in mature stands. Yet limited research has investigated fuel management treatments in young stands which can be particularly vulnerable to even low intensity fire. To address this gap, we investigated how prescribed fire (conducted in both the spring and fall) and pre-treatment fuel modifications impacted individual tree damage (or injury) and mortality in nine 13–14year old mixed conifer stands in the central Sierra Nevada, California, USA. Prior to burning, randomly selected trees were pruned to 1.8m height above ground. Randomly selected trees were also raked to reduce surrounding surface fuel. Raking fuel from trees reduced the amount of crown volume scorched for all species, but not enough trees died to determine if raking influenced probability of survival. Pruning was associated with reductions in crown consumption height and percent crown volume consumed (5% of crown volume in pruned versus <1% of crown volume unpruned, p=0.02) but was not a significant predictor of percent of crown volume scorched brown. Pruned trees had only 27% of the mortality of unpruned trees when less than half of the crown was scorched (p=0.046). However the mortality of pruned trees showed no less sensitivity to fire damage at higher levels of crown scorch (p=0.0076); in fact extremely scorched trees appeared more likely to die when pruned. Mortality differed strongly by species; giant sequoia (Sequoiadendron giganteum) showed 97% post-fire survival while nearly half of sugar pines (Pinus lambertiana) died. Fires occurring in spring months killed seven times as many trees as those in autumn months. While the results demonstrate that prescribed burns can be feasible in stands as young as 13years old, the factors of post-fire damage and mortality are different compared to mature stands. Species composition, pre-fire fuel reduction treatments, and timing of prescribed burns are all important considerations for managers wanting to develop resilient young stands with prescribed fire.

Effects of prescribed burning on ecophysiological, anatomical and stem hydraulic properties in Pinus pinea L.

Prescribed burning (PB) is a widespread management technique for wildfire hazard abatement. Understanding PB effects on tree ecophysiology is key to defining burn prescriptions aimed at reducing fire hazard in Mediterranean pine plantations, such as Pinus pinea L. stands. We assessed physiological responses of adult P. pinea trees to PB using a combination of dendroecological, anatomical, hydraulic and isotopic analyses. Tree-ring widths, xylem cell wall thickness, lumen area, hydraulic diameter and tree-ring δ(13)C and δ(18)O were measured in trees on burned and control sites. Vulnerability curves were elaborated to assess tree hydraulic efficiency or safety. Despite the relatively intense thermal treatment (the residence time of temperatures above 50 °C at the stem surface ranged between 242 and 2239 s), burned trees did not suffer mechanical damage to stems, nor significant reduction in radial growth. Moreover, the PB did not affect xylem structure and tree hydraulics. No variations in (13)C-derived water use efficiency were recorded. This confirmed the high resistance of P. pinea to surface fire at the stem base. However, burned trees showed consistently lower δ(18)O values in the PB year, as a likely consequence of reduced competition for water and nutrients due to the understory burning, which increased both photosynthetic activity and stomatal conductance. Our multi-approach analysis offers new perspectives on post-fire survival strategies of P. pinea in an environment where fires are predicted to increase in frequency and severity during the 21st century.

Phenotypic plasticity of post-fire activity and thermal biology of a free-ranging small mammal

Ecosystems can change rapidly and sometimes irreversibly due to a number of anthropogenic and natural factors, such as deforestation and fire. How individual animals exposed to such changes respond behaviourally and physiologically is poorly understood. We quantified the phenotypic plasticity of activity patterns and torpor use – a highly efficient energy conservation mechanism – in brown antechinus (Antechinus stuartii), a small Australian marsupial mammal. We compared groups in densely vegetated forest areas (pre-fire and control) with a group in a burned, open habitat (post-fire). Activity and torpor patterns differed among groups and sexes. Females in the post-fire group spent significantly less time active than the other groups, both during the day and night. However, in males only daytime activity declined in the post-fire group, although overall activity was also reduced on cold days in males for all groups. The reduction in total or diurnal activity in the post-fire group was made energetically possible by a ~3.4-fold and ~2.2-fold increase in the proportion of time females and males, respectively, used torpor in comparison to that in the pre-fire and control groups. Overall, likely due to reproductive needs, torpor was more pronounced in females than in males, but low ambient temperatures increased torpor bout duration in both sexes. Importantly, for both male and female antechinus and likely other small mammals, predator avoidance and energy conservation – achieved by reduced activity and increased torpor use – appear to be vital for post-fire survival where ground cover and refuges have been obliterated.

Do Wildfires Promote Woody Species Invasion in a Fire-Adapted Ecosystem? Post-fire Resprouting of Native and Non-native Woody Plants in Central Argentina.

We asked whether prescribed fire could be a useful management tool to reduce invasion by non-native plants in an ecosystem where native plants are supposed to be adapted to fires. Specifically, we compare the post-fire resprouting response of native and non-native woody species in Chaco Serrano forest of central Argentina. The measurements were carried out in five burnt areas where we selected ten native and seven non-native species. Our response variables were (1) post-fire survival, (2) types of resprouts, and (3) the growth of the resprouts. Our main results show that one year after the fire, survivals of native and non-native species were 0.84 and 0.89, respectively, with variances in survival seven times smaller in the native species group. Type of resprout was also less variable in native species, while growth of the resprouts was similar in native and non-native groups. We interpret that in most cases, the burning a forest with mixed native and non-native plants through prescribed fires will not differentially stop the invasion by non-native woody species even in ecosystems which are presumed to be relatively resistant to fires such as our study area.

The importance of mammalian torpor for survival in a post-fire landscape.

Wildfires have increased in frequency and intensity worldwide with climate change as a main driving factor. While a number of studies have focused on population changes in regard to fires, there are essentially no quantitative data on behavioural and physiological adjustments that are vital for the persistence of individuals during and after fires. Here we show that brown antechinus, a small insectivorous marsupial mammal, (i) endured a prescribed fire in situ, (ii) remained in their scorched home range despite unburned areas nearby, and (iii) substantially increased post-fire torpor use and thus reduced foraging requirements and exposure to predators. Hence, torpor is a physiological adaptation that, although not quantified in this context previously, appears to play a key role in post-fire survival for this and other heterothermic species.

Growth and reproductive post-fire responses of two shrubs in semiarid Patagonian grasslands

Variation in fire intensity affects the post-fire survival and growth of shrubs. We examined effects of maximum fire temperature (a proxy for fire intensity) on the survival, growth and reproductive performance of the shrubs Mulinum spinosum and Senecio bracteolatus at 1 and 2 years post-fire in north-west Patagonian grasslands. We applied two fire temperature treatments to plants of each species. All M. spinosum plants survived fire treatments but high fire temperature notably decreased survival of S. bracteolatus. Bud position and plant architecture probably influenced shrub survival. During the first growing season, M. spinosum did not produce seeds whereas S. bracteolatus plants burned at low temperature produced three times more seeds and bigger seeds than unburned plants. Also, seeds from burned plants of S. bracteolatus had higher germinability than seed from unburned plants. High survival and resprouting capacity of M. spinosum even after high fire intensity indicate that this species might be less affected by changes in fire regime.

Effects of high-severity fire drove the population collapse of the subalpine Tasmanian endemic conifer Athrotaxis cupressoides.

Athrotaxis cupressoides is a slow-growing and long-lived conifer that occurs in the subalpine temperate forests of Tasmania, a continental island to the south of Australia. In 1960-1961, human-ignited wildfires occurred during an extremely dry summer that killed many A. cupressoides stands on the high plateau in the center of Tasmania. That fire year, coupled with subsequent regeneration failure, caused a loss of ca. 10% of the geographic extent of this endemic Tasmanian forest type. To provide historical context for these large-scale fire events, we (i) collected dendroecological, floristic, and structural data, (ii) documented the postfire survival and regeneration of A. cupressoides and co-occurring understory species, and (iii) assessed postfire understory plant community composition and flammability. We found that fire frequency did not vary following the arrival of European settlers, and that A. cupressoides populations were able to persist under a regime of low-to-mid severity fires prior to the 1960 fires. Our data indicate that the 1960 fires were (i) of greater severity than previous fires, (ii) herbivory by native marsupials may limit seedling survival in both burned and unburned A. cupressoides stands, and (iii) the loss of A. cupressoides populations is largely irreversible given the relatively high fuel loads of postfire vegetation communities that are dominated by resprouting shrubs. We suggest that the feedback between regeneration failure and increased flammability will be further exacerbated by a warmer and drier climate causing A. cupressoides to contract to the most fire-proof landscape settings.

Isotopic and anatomical signals for interpreting fire-related responses in Pinus halepensis

Our study aims to define isotopic and anatomical responses to fires of P. halepensis . Main results: decrease in tree growth and relative conductivity and increase in water use efficiency. We investigated the ecophysiological responses of a Pinus halepensis Mill. stand surviving two wildfires in southern France. Basal area, isotope composition and anatomical traits were analysed before and after fires, using tree rings to assess the ecological responses of trees to heat-related damage. The years were determined based on the presence of fire scars. Stable isotopes (δ13C and δ18O) were measured in tree rings before and after the “fire years”. Anatomical observations allowed qualitative analysis of the scar region and quantification of tracheid size in tree rings before and after the fire years. Relative and percentage conductivity of earlywood and latewood far from the woundwood were estimated. Results showed a decrease in tree growth after the fire events accompanied by an increase in 13C-derived water use efficiency (WUEi) and a decrease in relative conductivity. The positive relationship between δ13C and δ18O suggested that both isotopic variations are mostly driven by changes in stomatal conductance following fire events. P. halepensis proved to be a strong isohydric species, able to survive frequent fires with temporary ecophysiological modifications and anatomical adaptations. Our findings afford new insights into post-fire survival strategies of this species in an environment where fires are predicted to increase in frequency during the twenty-first century.

Post-fire survival and regeneration of Eucalyptus globulus in forest plantations in Portugal

Eucalyptus globulus is one of the most important pulpwood plantation species in the world, and nowadays it is present in most continents. Some of the regions where E. globulus plantations have been expanding have a high incidence of wildfires; therefore, knowing the factors affecting the fire resistance and resilience of this species is particularly important for forest management. This is the case of Portugal, where nearly 1.4million ha of land burned in the last decade and where E. globulus has become the most widespread tree species. In this paper we assessed the short-term post-fire responses of E. globulus trees in four sites affected by wildfires, and investigated their potential relationships with fire severity descriptors and with tree and stand characteristics. One year after fire, individual tree mortality was low (3.9%) but most trees (79.2%) were top-killed (i.e. had stem mortality). Both post-fire tree mortality and top-kill increased with fire severity (expressed by maximum char height and/or char severity rating). Moreover, top-kill was positively related with vegetation cover and the proportion of pine trees in the stand, and was negatively related with tree diameter and the proportion of other broadleaved trees in the stand. The most common post-fire regeneration type among the sampled trees was basal resprouting (89.2%), though 20.9% had epicormic resprouting. The number of basal resprouts increased with char height, top-kill and diameter of the parent tree, and decreased with slope. In terms of post-fire growth, the height of the dominant resprout increased with the number of resprouts and with char severity, while it decreased in drier southern aspects. Results showed that E. globulus is a fire-resilient species with a very high probability of surviving fire; however, in forest plantations where trees are usually felled in short rotations (thus with limited size), individuals have a high probability of being top-killed. The presented models may be useful to help managers on the assessment of post-fire production losses and regeneration potential in E. globulus plantations.

Post-fire recovery occurs overwhelmingly by resprouting in the Chaco Serrano forest of Central Argentina

In fire-prone landscapes, differences in post-fire regeneration by resprouting between species or sites could be far more important in explaining vegetation physiognomy and composition than seed regeneration. This is the first study exploring the relative contribution of tree resprouts and seeds to post-fire crown volume in the Chaco Serrano forest of South America. Additionally, we compare the resprouting response among species and quantify post-fire changes in tree composition among sites. We established 290 permanent plots distributed in three sites affected by wildfires in 2005. For all tree species in all plots we recorded survival of every individual 1 year after the fire; at the plot level, we recorded the above-ground tree volume before and 3 years after the fire. Resprouting from the base was the main resprouting type. Survival varied between species from 73% to 100% for native species and from 7% to 100% for non-natives. Before the fire, crown volume was similar in the three sites, and was completely lost after the fire in 92% of the plots. Three years after the fire, between 8% and 58% of crown volume was recovered. The ratio of crown recovery because of resprouts and seedlings was 1562:1. Tree composition exhibited few changes because of the high post-fire survival of most native species. We conclude that in the semiarid Chaco Serrano ecosystem tree species regenerate mainly by resprouting. This regeneration mode should be taken into account to better understand post-fire successional pathways of these forests, their management and the restoration of burnt forest areas.

Fire Injury Severity in an Eastern Sierra Nevada Mixed Conifer Stand: Variability and Influencing Factors

Variability of postfire injury and stand and individual tree factors that affected survival responses of eastern Sierra Nevada conifers to wildfire were examined. Prefire measurements served as a basis of comparison for postfire conditions in a mixed conifer stand located in the eastern portion of the Lake Tahoe Basin and provided insight into predisposing influences on survival. Species composition consisted primarily of Jeffrey pine (Pinus jeffreyi Grev. & Balf.) and California white fir (Abies concolor var. lowiana [Gord.] Lemm.) along with a minor component of sugar pine (Pinus lambertiana Dougl.). Postfire survival was higher in Jeffrey pine than white fir but was highest overall in sugar pine. Catastrophic crown loss occurred less frequently in Jeffrey pine than in the fir but was least common in sugar pine. Survival generally increased with tree size, but this relationship did not extend to the largest trees in the stand. Among an array of regression models used to evaluate selected variables for their predictive capacity regarding postfire survival, prefire stand density was found to negatively influence that of Jeffrey and sugar pine, and survival of Jeffrey pine and white fir was negatively correlated with bole char. These results provide natural resource managers guidance in the selection of viable trees for retention during the salvage harvesting operations that often follow wildfire events.

Studying the post-fire performance of tussock grasses in Patagonia: Survival, biomass production and early competition

Patagonia grasslands are subjected to two main disturbances, fire and grazing, but little information is available about its effects on vegetation. We studied post-fire survival and resprouting ability of two dominant grass species, Stipa speciosa and Festuca pallescens, for four years; evaluated the effect of early post-fire defoliation on both species; and tested whether competition is important in post-fire recovery in San Ramón Ranch, NW Patagonia (Argentina). To simulate grazing, a clipping treatment was applied at the beginning and end of growing seasons. Survival rates were high (>60%) and, after three years, biomass of both species in the burned area was similar to the unburned area. Competition seems to play an important role in the early post-fire recovery of both species, particularly in the case of F. pallescens that increased 87% the biomass production without competition. Fire may improve forage quality by eliminating the standing dead material, but early post-fire grazing might endanger the persistence of F. pallescens. We suggest requiring a resting period before livestock introduction and controlling grazing intensity.

Regional and local effects on reproductive allocation in epicormic and lignotuberous populations of Banksia menziesii

Reproductive allocation (RA) is a measure of how resources (biomass, nutrients) are partitioned between reproductive structures and the rest of the plant. For plants that resprout after fire, the percentage of resources allocated to reproduction may vary depending on their resprouting ability. Our study examines the percentage RA (biomass, N, P, K) and nutrient content of current season’s growth in southern (Swan Coastal Plain) epicormic and northern (Eneabba Plain) lignotuberous resprouter populations of Banksia menziesii (Proteaceae), a species endemic to nutrient-impoverished sandplains of southwestern Australia. Within each population, plants along road edges were compared with plants not associated with road edges. There was no difference in total nutrient content of current year’s growth between both resprouting types, except that total K in the shoots of lignotuberous populations was >2 times that in the epicormic populations. Non-road lignotuberous plants allocated twice the biomass, N and P, and 13.5 times the K, to reproduction as non-road epicormic plants. Lignotuberous populations had the highest RA (17–34% of biomass, N, P, K), with non-road epicormic populations the lowest RA (3–15%). This can be viewed as an adaptive (ultimate) response to the poorer postfire survival and recruitment conditions where the lignotuberous populations occur. Total biomass and nutrient content of road-edge plants was 2–3 times that of non-edge plants. Lignotuberous populations in both road positions allocated the same fraction of biomass, N and P to reproduction, whereas road-edge populations allocated 10% less K than non-road. Road-edge epicormic populations allocated 5–10% more biomass, N, P and K to reproduction than non-road populations. This can be viewed as an ecophysiological (proximate) response to the better growing conditions created by the roadways that may also ultimately have an adaptive explanation.

Tree Mortality Following Boreal Forest Fires Reveals Scale-Dependant Interactions Between Community Structure and Fire Intensity

Fire disturbance patterns influence forest communities at a range of spatial scales. Forest community structure may also influence fire disturbance patterns, because tree species vary in their fuel value and in their tolerance to fire damage. However, the influence of community structure on fire disturbance likely depends on latent ecological differences between fires and on the spatial scale at which patterns are observed. Using data on fire intensity, community structure, and post-fire tree survival in four systematically sampled boreal forest fires, we tested the hypotheses that: (1) patterns in post-fire tree survival reflect interactions between fire intensity and community structure; (2) these relationships change with the spatial scale of observation. To test the first hypothesis, we used information theoretic methods to compare eight generalized linear mixed effects models describing the influence of community structure and fire intensity on tree survival in a 500 m2 sample plot, accounting for latent fire-to-fire differences in response. To test the scaling hypothesis, we reaveraged the data at nine successively larger spatial resolutions up to approximately 2 km2, at each resolution tracking the parameter values of the best model. When fit to the plot-level data, the dominant feature of the best model was a strong intensity–survival correlation which varied from fire to fire, and depended on plot-level community structure. In some fires, community structure and survival became more tightly coupled at larger scales, whereas fire intensity became less important. These results support the view that fire disturbance patterns are influenced by cross-scale interactions between community structure and fire intensity.

Post-fire survival and flushing in three Sierra Nevada conifers with high initial crown scorch

With growing debate over the impacts of post-fire salvage logging in conifer forests of the western USA, managers need accurate assessments of tree survival when significant proportions of the crown have been scorched. The accuracy of fire severity measurements will be affected if trees that initially appear to be fire-killed prove to be viable after longer observation. Our goal was to quantify the extent to which three common Sierra Nevada conifer species may ‘flush’ (produce new foliage in the year following a fire from scorched portions of the crown) and survive after fire, and to identify tree or burn characteristics associated with survival. We found that, among ponderosa pines (Pinus ponderosa Dougl. ex. Laws) and Jeffrey pines (Pinus jeffreyi Grev. &amp; Balf) with 100% initial crown scorch (no green foliage following the fire), the majority of mature trees flushed, and survived. Red fir (Abies magnifica A. Murr.) with high crown scorch (mean = 90%) also flushed, and most large trees survived. Our results indicate that, if flushing is not taken into account, fire severity assessments will tend to overestimate mortality and post-fire salvage could remove many large trees that appear dead but are not.

The Role of Nurse Trees in Mitigating Fire Effects on Tropical Dry Forest Restoration: A Case Study

The threat of fire is always a consideration when establishing a forest restoration program. Two wildfires occurred in 2006 and 2007 in an established dry forest restoration project in Puerto Rico. The original goal of the project was to determine differential growth responses of native trees under the nurse tree Leucaena leucocephala versus in open sites. Tree species growth, mortality and response to the fires were evaluated according to their leaf habit, successional status, and prefire tolerance to environmental conditions. Results showed that regardless of a species' leaf habit and successional status, trees attained greater height and lower mortality under nurse trees. In open sites, sprouting was the most common fire response and mature-forest and evergreen species had greater postfire survival than pioneers and deciduous species. Although nurse trees are typically used to help manage nutrient or light environments in reforestation projects, these trees also appear to provide a secondary benefit of limiting fire damage by reducing fuel load.

Optimising the management of Pinus nigra Arn. stands under endogenous risk of fire in Catalonia

The present study considers the risk of fire as an endogenous component in a stand management optimisation problem for even-aged Pinus nigra Arn. stands in Catalonia (North-east Spain). A simulation-optimisation system, RODAL, was used to determine the optimal stand management when maximal soil expectation value (SEV) was the objective. This system was modified to include fire occurrence and post-fire tree survival models. The fire occurrence and post-fire survival models used in the study have been developed for forest planning purposes using explanatory variables that relate the probability of fire occurrence and the expected loss to such stand variables which can be affected by management actions. Non-linear stochastic optimisation was used to find the optimal stand management schedule. The rotation length as well as the timing and intensity of thinnings were optimised. The inclusion of the risk of fire had a clear effect on the optimal stand management schedule, especially on the regime of thinnings which tended to include early thinnings as means of reducing the risk and expected loss. The regeneration cuts tended to be earlierwith increasing fire risk.

CAVITATION RESISTANCE AMONG 26 CHAPARRAL SPECIES OF SOUTHERN CALIFORNIA

Resistance to xylem cavitation depends on the size of xylem pit membrane pores and the strength of vessels to resist collapse or, in the case of freezing-induced cavitation, conduit diameter. Altering these traits may impact plant biomechanics or water transport efficiency. The evergreen sclerophyllous shrub species, collectively referred to as chaparral, which dominate much of the mediterranean-type climate region of southern California, have been shown to display high cavitation resistance (pressure potential at 50% loss of hydraulic conductivity; P50). We examined xylem functional and structural traits associated with more negative P50 in stems of 26 chaparral species. We correlated raw-trait values, without phylogenetic consideration, to examine current relationships between P50 and these xylem traits. Additionally, correlations were examined using phylogenetic independent contrasts (PICs) to determine whether evolutionary changes in these xylem traits correlate with changes in P50. Co-occurring chaparral species widely differ in their P50 (� 0.9 to � 11.0 MPa). Species experiencing the most negative seasonal pressure potential (Pmin) had the highest resistance to xylem cavitation (lowest P50). Decreased P50 was associated with increased xylem density, stem mechanical strength (modulus of rupture), and transverse fiber wall area when both raw values and PICs were analyzed. These results support a functional and evolutionary relationship among these xylem traits and cavitation resistance. Chaparral species that do not sprout following fire but instead recruit post-fire from seed had the greatest resistance to cavitation, presumably because they rely on post-fire survival of seedlings during the summer dry period to persist in the landscape. Raw values of hydraulic vessel diameter (dh), maximum vessel length, and xylem-specific hydraulic conductivity (Ks) were correlated to P50; however, dh, maximum vessel length, and Ks were not correlated to P50 when analyzed using PICs, suggesting that these traits have not undergone correlated evolutionary change. We found no difference in xylem traits between species occurring at freezing vs. nonfreezing sites, although freezing has been shown to affect the survival and distributions of some chaparral species. Stem mechanical strength, fiber properties, and post-fire regeneration type appear to be key factors in the evolution of cavitation resistance among chaparral shrubs.